What defines difficult genomic regions, and can D3 assay design be used for those regions?

Regions bearing greater than 65% guanine-cytosine content can be difficult to design against. Sequences are filtered based on GC content. If an assay bearing less than 65% GC content cannot be derived, this is reported as a "no design." Successfully designed sequences that have greater than 65% GC content will be assigned a medium design rank automatically.